A conventional water electrolyzer, which electrolyzes raw water in an electrolyzing section to alkali water and acid water by supplying electric power to electrodes in the electrolyzing section, has been known. An example is disclosed in Japanese Patent Publication No. 60-20073. The water electrolyzer disclosed in the Japanese Patent has an electrolyzing section whose inner space is divided into a cathode chamber and an anode chamber by a porous partition. There is provided an electrode in each chamber so as to electrolyze raw water. Alkali water is produced in the cathode chamber; acid water is produced in the anode chamber. The proper pH value of alkali water for drinking is 8.5-9.5. In the case of less than pH 8.5, electrolyzed water is almost the same as raw water. On the other hand, in case of more than pH 10.0, it is harmful for human bodies to drink. Therefore, in the conventional water electrolyzer, fixed electric power based on the capacity, etc. of the electrolyzing section is supplied to the electrodes to maintain the pH value of the alkali water between 8.5-9.5 pH.
However, the conventional method of maintaining the pH value has the following disadvantage.
Fixed electric power supplied to the electrodes to maintain the pH value in the prescribed range is defined under the condition that the electrolyzing section is filled with raw water. If the electrolyzing section is not filled with raw water and the fixed electric power is supplied to the electrodes, the electrlyzing capability is too high and the pH value will be in the harmful range: greater than 9.5.